Avoiding Ties in Shortest Path First Routing

Executive Summary

First the authors discuss problems associated with ties and flow splitting with shortest path first protocols such as OSPF and IS-IS. The problems relate to uncertainty in the splitting when there is multiple shortest path from a source to a destination. Even if routers are configured for even splitting, there can easily be unpredicted biases that can overload links and thereby affect quality of service guarantees for virtual leased lines. Second they show how one can set the OSPF/IS-IS weights so as to avoid ties and yet minimize congestion. On real and synthetic networks they demonstrate experimentally that load balancing typically can be done nearly as well without ties as with ties assuming exact even splitting.